Percussion drill bits are widely used both for drilling relatively shallow bores in hard rock and for creating deep boreholes. For the latter application, drill strings are typically used in which a plurality of rods are added to the string via coupling sleeves as the depth of the hole increases. A terrestrial machine is operative to transfer a combined impact and rotary drive motion to an upper end of the drill string whilst a drill bit positioned at the lower end is operative to crush the rock and form the boreholes. Fluid is typically flushed through the drill string and exits at the base of the borehole via apertures in the drill head to flush the drill cuttings from the boring region to be conveyed backward and up through the bore around the outside of the drill string. Example percussive drill bits are disclosed in U.S. Pat. No. 3,388,756; GB 692,373; RU 2019674; US 2002/0153174; U.S. Pat. No. 3,357,507, US 2008/0087473; and WO 2009/067073.
The drill bit typically comprises a drill head that mounts a plurality of hard cutting inserts, commonly referred to as buttons. Such buttons comprise a carbide based material to enhance the lifetime of the drill bit. In particular, WO 2006/033606 discloses a rock drill bit having a head with a plurality of peripheral gauge buttons distributed circumferentially at an outer perimeter of the drill head. The gauge buttons are configured to engage material to be crushed and to determine the diameter of the borehole. The head also mounts a plurality of front buttons provided at a recessed front face of the drill head for engaging material to be crushed at the axial region immediately in front the drill head. WO 2008/066445; U.S. Pat. No. 3,955,635 and WO 2012/174607 also disclose drill bits having a plurality of peripheral gauge buttons distributed circumferentially at an outer perimeter of the head with a plurality of front buttons distributed over the front face.
Typically, a plurality of flushing channels or grooves are recessed into the drill head to allow the flushing of fractured material rearwardly from the drill bit via the flushing fluid. However, convention drill heads are disadvantageous in that large pieces of material cut from the seam cannot pass through the flushing grooves without being further crushed by the bit head. This reduces the effectiveness of the cutting bit to fracture and further penetrate the rock or seam face. What is therefore required is an improved percussive drill bit that is optimised to allow relatively larger pieces of cut material to pass rearwardly from the bit head whilst maximising the cutting action.